I think I may have isolated what is causing the error.
If you wrap the server form tag in a <div> it causes the error. For example this simple page caues the error:
<%
@ Page Language="C#" %>
<!
DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<
html xmlns="http://www.w3.org/1999/xhtml">
<
head runat="server">
<title>Untitled Page</title>
</
head>
<
body>
<div>
<form id="form1" runat="server">
<CuteWebUI:UploadAttachments InsertText="Upload Multiple files Now" runat="server" ID="Attachments1" />
</form>
</div>
</
body>
</
html>